178 THE CELL 



Schwann (I. 31), whilst, adopting- Schleiden's theory, fell into a 

 second, and still greater error. He considered that the young 

 cells developed, not only within the mother-cell (as propounded 

 by Schleiden), but also outside of it, in an organic substance, which 

 is frequently present in animal tissues as intercellular substance, 

 and which he called also Cytoblastem. Thus Schwann taught 

 that cells were formed spontaneously both inside and outside of 

 the mother-cell, which would be a genuine case of spontaneous 

 generation from formless germ substance. 



These were indeed grave fundamental errors, from which, how- 

 ever, the botanists were the first to extricate themselves. In the 

 year 1846 a general law was formulated in consequence of the 

 observations of Mohl (VI. 47), Unger, and above all, Nageli 

 (VI. 48). This law states, that new plant cells only spring from 

 those already present, and further that this occurs in such a 

 manner, that the mother-cell becomes broken up by dividing into 

 two or more daughter-cells. This was first observed by Mohl. 



It was much more difficult to disprove the theory, that the cells 

 of animal tissues arise from cytoblasts. and this was especially the 

 case in the domain of pathological anatomy, for it was thought 

 that the formation of tumours and pus could be traced back to 

 cytoblasts. At last, after many mistakes, and thanks to the 

 labours of many investigators, amongst whom Kolliker (VI. 

 45, 46), Reichert (VI. 58, 59), and Remak (VI. 60, 61) must be 

 mentioned, more light was thrown upon the subject of the genesis 

 of cells in the animal kingdom also, until finally the cytoblastic 

 theory was absolutely disproved by Virchow, who originated the 

 formula, " Omnis cellula e cellula" No spontaneous generation of 

 cells occurs either in plants or animals. The many millions of 

 cells of which, for instance, the body of a vertebrate animal is 

 composed, have been produced by the repeated division of one 

 cell, the ovum, in which the life of every animal commences. 



The older histologists were unable to discover what part the 

 nucleus played in cell-division. For many decades two opposing 

 theories were held, of which now one and now the other obtained 

 temporarily the greater number of supporters. According to the 

 one theory, which was held by most botanists (Reichert VI. 58 ; 

 Auerbach VI. 2a, etc.), the nucleus at each division was sup- 

 posed to break up and become diffused throughout the protoplasm, 

 in order to be formed anew in each daughter-cell. According to 

 the other (C. E. v. Baer ; Joh. Miiller ; Remak VI. 60; Leydig; 



